Device for detaching mercury drops from a capillary tube



Dec. 14, 1965 H. M DONALD DAVIS ETAL 3,222,942

DEVICE FOR DETACHING MERCURY DROPS FROM A CAPILLARY TUBE Filed Jan. 9.1961 FIG.- 1.

United States Patent 3,222,942 DEVICE FOR DETACHING MERCURY DROPS FROM ACAPILLARY TUBE Herbert MacDonald Davis, Barnehurst, and WilliamHartshorn Lockwood, Pinner, England, assignors to United Kingdom AtomicEnergy Authority, London, England Filed Jan. 9, 1961, Ser. No. 81,466Claims priority, application Great Britain, Jan. 16, 1960, 1,655/ 60 4Claims. (Cl. 74102) This invention relates to devices for detachingmercury drops from the end of a capillary tube. Such devices are used inpolarography to detach the drops from dropping mercury electrodes. Thepresent invention has particular application in differentialpolarography, in which it is important that the mercury drops from twosuch electrodes should be detached as nearly simultaneously as possible,and that the period between successive drops should be as constant aspossible.

According to the present invention a device for detaching mercury dropsfrom a capillary tube comprises a moveable mounting for the tube, stopmeans for limiting the movement of the mounting between two positions,an electromagnet having a moveable armature, the armature being loadedto return to an unenergised position, a lost motion coupling between thearmature and the mounting for moving the mounting from one position tothe other when the electromagnet is energised, and means for restoringthe mounting to said one position.

The moveable mounting may comprise an arm pivoted about a substantiallyvertical axis, one end of the arm being adapted to hold a capillary tubein a substantially vertical position and the other end being locatedbetween two stops, the lost-motion coupling being between the armatureand said other end of the arm and the restoring means comprisingspring-loading means acting on said other end.

The lost-motion coupling may comprise a pin connected to the armatureand an apertured member con nected to the arm, the pin passing throughthe aperture and having a head adapted to engage the member, the headbeing spaced apart from the member when the armature is in itsunenergised position. The springloaded means acting on said other end ofthe arm may comprise two springs acting in opposite directions.

The electromagnet may comprise a solenoid, the armature being aspring-loaded rod arranged to move axially within the solenoid. Theelectromagnet may comprise a core portion coaxial with the armaturehaving a nonmagnetic end-portion which abuts the armature when thearmature is drawn into the coil.

Said one end of the arm may be bifurcated into two branches each adaptedto hold a capillary tube.

To enable the present invention to be more readily understood, attentionis directed, by way of example, to the accompanying drawings, wherein:

FIG. 1 shows a part sectional plan view and FIG. 2 shows an elevation ofan embodiment of the invention.

Referring to FIG. 1, an arm 1 pivoted about a roller bearing 2 mountedon a bracket 7, is bifurcated at one end into two branches 3 and 4, eachof which carries a clamp 5 whose jaws are recessed at 6 to hold acapillary tube (not shown) in a vertical position. The mercury is fed toeach capillary tube from a separate reservoir. The other end of the armis located between adjustable screw stops 8 and 9 which limit themovement of the arm about the bearing 2. The stops are mounted on anangle-section base-plate 10 which is secured to the bracket 7 by boltswhich pass through a vertical polarograph stand 11, and also serve tosecure the device to the stand. As

3,222,942 Patented Dec. 14, 1965 an indication of the dimensions, thebase-plate is 6 inches long and the drawings are to scale.

In the quiescent state, the position of the arm 1 is such that centerstabling springs 29 and 30 are in their natural condition (both lightlycompressed to the same extent). The return stop 8 is adjusted to barelytouch the arm 1, and the forward stop 9 is adjusted to allow the arm 1to undergo a small movement.

Fastened to the arm 1 is a bracket 12 comprising a fork-like verticalmember 13, through the aperture in which passes a pin 14 having a head15 adapted to engage the member 13. The pin 14 is adjustably fastened toa soft iron rod 16 constituting an armature, which slides in a brasstube 17 on part of which is wound a solenoid 18. The solenoid forms partof an electromagnet comprising a tubular soft iron housing 19 having anend-cap 20, and a threaded core-portion 21 which is a screw fit in theend-cap 20 and is secured by a lock-nut 22. On the inner end of thecore-portion 21 is a small brass projection 23 which abuts the end ofthe armature 16 when the latter is drawn into the solenoid.

In the unenergised state the position of the armature 16 is determinedby spring-loading means comprising a coil spring 24 (not shown inFIG. 1) which encircles the armature and holds a washer 25 on the end ofthe armature 16 in contact with a bracket 26 having a vertical forkedportion 27 through which passes the pin 14. The bracket 26 is fastenedto a clamp 28 which secures the electromagnet to the base-plate 10. Thebracket 26 is made of steel and also helps to complete the magneticcircuit.

In the quiescent state, i.e. with the electromagnet unenergised, the arm1 is barely touching the return stop 8 and the armature 16 is heldagainst the bracket 26 as described. The device is so adjusted that inthis state the head 15 of pin 14 is spaced apart from the aperturedmember 13 of bracket 12 (as shown in FIG. 1), so that a lost-motioncoupling is provided between the armature and the arm, i.e. the couplingis such that motion of the armature from its unenergised position is notcommunicated to the arm until the armature has moved some distance. As aresult of this lost motion, when the electromagnet is energised thearmature has acquired considerable momentum before head 15 engagesmember 13; thereafter member 13 travels with head 15, and the arm 1 ismoved rapidly to a position against the forward stop 9. The resultinginertial and viscous forces acting in shear detach the mercury dropsfrom the two capillary tubes. When finally adjusted, the clearancebetween head 15 and member 13 in the quiescent state is about 0.25 inchand the arm movement between stops 8 and 9 is less than 0.1 mm.(exaggerated in the drawing), the latter distance being adjusted to theminimum which gives reliable detachment of the drops.

The projection 23 helps to prevent the armature being held in byresidual magnetism when the electromagnet is de-energised. Sufiicientclearance is provided between the armature 16 and the tube 17 to preventair-damping during inward movement of the armature. The arm 1 is made ofaluminium alloy to reduce its inertia.

The center stabling springs 29 and 30 restore the arm 1 to its quiescentposition against return stop 8 without a sudden movement which mightdetach a second mercury drop. The also prevent rebound of the arm.

The solenoid has 550 turns of 24 standard wire gauge (0.022 inchdiameter) copper wire and is designed to be energized by 12 volt pulsesof 0.02 second duration, obtained from the differential polarographcircuit.

In one such polarograph the period between successive pulses is 7seconds, the levels of the mercury reservoirs being adjusted to giveboth capillary tubes approximately the same natural dropping period(i.e. by gravity alone) in the range 1012 seconds. The two tubes areabout 4 /2 inches long and are selected to have matched bore diametersin the range '.O5 -0.1 mm.

The precise timing of the present dropping device results from thelost-motion coupling between the armature and the arm; together with theshort travel of the arm. When a voltage is appliedto an inductivecircuit the current rises exponentially with time, so that the'magneticforce acting on the armature also rises. from zero. A rigidcouplingWOllldlhBlBfOlG result in the capillary tubes being moved with'the lowinitialvelocityof the armature, leading to imprecision in the droppingaction] In the present device the armature acquires considerablemomentum before movement is communicated to the arm, and the' capillarytubes. travel at a very high velocity over a very short: distance, thus:defining closely the instantiof'detachment. The small movement of thecapillary tubes also'avoid disturbing the solutions in which theyareimmersed, Precision of the timingbetween suecessive drops is alsoimproved by keeping the time-constant (L/R) of the solenoid as lowaspracticable, in the present case about 0.01 second, so' that the forceon' the armature rises rapidly after the/initiation of'the current pulseto the solenoid.

We claim:

1. A device for detaching mercury drops from a capillary tube comprisinga moveable'arm including a mounting for holding a capillary tube, anelectromagnet'having armature moveable between an unenergized and anenergized'position, a lost-motion coupling coupling said armature tosaid arm, thecoupling servingto delay move ment of the arm until afterthe armature has moved a predeterminedamount,first loading meansactingon said armature to return it to the unenergized position, twosubstantially non-resilientistops defining the limits of the movement ofsaidarrn,andsecond-loading means acting a vertical position, meansdefiningzaverticalaxis about.

which said arm.is pivoted, anelectromagnethavingan. armaturemoveablevbetween an'unenergized and an-energized position,- alost-motion coupling. couplingsaid arma-- ture to said arm, thetcouplingserving-to delaymovement of the arm until after the armature has moved apredeterminedamount, first loading means acting on said'armature toreturn it to the unenergized position, two substantially non-resilientstops defining the limits of the movement of said arm, andsecond loadingmeans acting on said arm to. restore said arm to-one of said limitsafter being moved therefrom tothe other of said limits by theenergization of said electromagnet.

3. A device for detaching mercury drops from a vertically mountedcapillary tube" comprising. a moveable arm; means to secure a capillarytube to said armadjacent one endof the arm and inavertic'al position;means defining a vertical axis about which said arm is pivoted;

an electromagnet having an armature moveable between an unenergized andan energized position; an apertured member secured to said arm, a pinsecured to said armature; the pin having-a head and passing through theaperture in said member, the head beingspaced apart from said' memberwhen said armature is in the'unenergized position and moveableonenergization oi the electromagnet to engage themember so that alost-motion coupling is-provided to transfer motion from said armatureto said armyfirstleading means acting on said-armature toreturn it tothe unenergized position; two substantially nonresilient stops definingthe limits of the movement of said arm; and second loading means actingon said arm to restore said arm to one ofsaid limits after being-movedtherefrom to the other of said limits by the energization of saidelectromagnet.

4. A device in accordance with claim-3' wherein said second loadingmeans comprises a pair of'center stabling helical springs acting on theother end'of said arm.

References Cited bylthe Examiner UNITED STATES PATENTS 905,136 12/1908'Barnum 317186 X 2,361,295 10/1944 Kanner et al. 324--3l 2,63 8,804 5/1953 Heinrich. 2,689,330 9/1954 Staunton 3243l 3,049,939 8/1962 Beard74-526 BROUGHTON G; DURHAM, Primary Examiner.

SAMUEL BERNSTEIN, FREDERICK M. STRADER,

Examiners;

1. A DEVICE FOR DETACHING MERCURY DROPS FROM A CAPILLARY TUBE COMPRISINGA MOVEABLE ARM INCLUDING A MOUNTING FOR HOLDING A CAPILLARY TUBE, ANELECTROMAGNET HAVING ARMATURE MOVEABLE BETWEEN AN UNENERGIZED AND ANENERGIZED POSITION, A LOST-MOTION COUPLING COUPLING SAID ARMATURE TOSAID ARM, THE COUPLING SERVING TO DELAY MOVEMENT OF THE ARM UNTIL AFTERTHE ARMATURE HAS MOVED A PREDETERMINED AMOUNT, FIRST LOADING MEANSACTING ON SAID ARMATURE TO RETURN IT TO THE UNENERGIZED POSITION, TWOSUBSTANTIALLY NON-RESILIENT STOPS DEFINING THE LIMITS OF THE MOVEMENT OFSAID ARM, AND SECOND LOADING MEANS ACTING ON SAID ARM TO RESTORE SAIDARM TO ONE OF SAID LIMITS AFTER BEING MOVED THEREFROM TO THE OTHER OFSAID LIMITS BY THE ENERGIZATION OF SAID ELECTROMAGNET.